ROOTS AND THEIR FUNCTIONS 81 
hoe are separated from each other, and thus 
the continuity of their surfaces with that of 
the lower soil is largely interrupted. At the 
same time the broken, loose soil serves to 
check evaporation, inasmuch as it shelters 
the lower unbroken (therefore continuous) soil 
both from the sun and from the drying 
influence of currents of air. It is a matter 
of common experience that if plants are 
grown in unwatered soil long enough, they 
begin to droop and wilt. This means that 
the root-hairs are not able to extract enough 
water from the ground to keep pace with that 
which is lost by the plant. Wilting takes 
place when the water contents of the soil fall 
below a certain amount, and this varies greatly 
in different soils, but is fairly constant for 
each particular kind. Thus, in sand a plant 
may utilise all the water down to about 1-2% ; 
while in heavy clay the water ceases to be 
available as soon as its content sinks below 
about 25 %. It is evident that there is 
probably some relation between the physical 
state of the soil, and its physiologically avail- 
able water content. And this turns out to 
be the case. The fine particles of clay, with 
their relatively enormous surface, retain far 
more water than sand with its large particles 
and relatively small surface. Ingenious ex- 
periments on soil in centrifugal machines have 
shown that approximately the same amount 
of force is required to clear out water from 
clay so as to leave 25 % remaining as is 
F 
